Price!! Size and weight is amazing for this zoom range. Build quality good. Overall image quality is good, even at 600mm. Image stabilization seems adequate. Did I mention price?? Yes, but I'll state it again.

Cons:

Autofocus servo does not function properly with two of my Canon camera bodies, the 7D and the MkIV. A relatively slow lens at f/6.3, but for the money, not so bad!

At this price I did not expect perfection, but I did expect it to function as advertised. It does not. The autofocus servo function did not work properly on my 7D and 1D Mark IV. From what I've read, I'm not alone in having this experience. In my brief time with this lens, the autofocus servo did appear to function properly on the 5D Mark III, but it was a bit sluggish--as one would expect for a lens that is f/6.3.

These are serious, unacceptable flaws in my judgment. Therefore, I can't recommend this lens at the present time. However, if Tamron corrects these flaws, this lens would represent surprisingly high quality at an excellent price and be stiff competition for other telephoto zooms. The lens already represents a breakthrough for a zoom lens reaching 600mm because of its size, weight, and price!